Motor Now that the frame is done we will do the motor. Obviously it is a RD400 motor…about 1978 and it's an E model with CDI ignition. Because we are going to be setting the squish clearance we need to make sure that the head is always centered on the barrel. The RD400 head has quite a bit of clearance on the cylinder studs. So to ensure that the head is always centered on the barrel we have to "Dowel the head" Here is how a very knowledgeable member of another Two Stroke board dowels the heads of a
HOW TO "DOWEL" THE HEADS BY "COYOTE"
If you want a tight squish clearance the heads have to be centered on the cylinder. The fix (for older engines at least) is to dowel the heads centered onto the cylinders. But what if you want to switch heads. The dowels will never line up, until now. I made a drill jig out of 5/16" gage stock (tool steel) and hardened it.
You can line up the center hole with the head relief diameter using your fingernail. Just bump it around till it's centered and then tighten the bolts. I'm holding the head with the mandrel used to turn the heads in a lathe. The mandrel is in a vise on the mill table. I'm using a 9/16" reamer. just go 5 mm deep into the head. Now you have to ream the cylinders, also about 5 mm deep. Make sure you get the handedness correct, you don't want them to be backwards. By the way I'm holding the cylinders in a jig I use to hold them in a lathe and bore them.
The magic of this method is that you can ream several sets of heads and they will all fit right on the cylinder and line up exactly. I have been using 9/16" diameter hollow dowels with about 0.003" clearance and maybe a 0.030" wall. They are 9 mm long and trapped in the "double blind" dowel holes.
Now I can change heads in just a couple of minutes to compare performance, or just to change performance. I also grind out the holes in the head gasket so it is lined up with the cylinder bore and trapped on the locating dowels. So that's how to dowel your heads.